Investigation of effectiveness of flow diverter stents and detecting stagnation region formation in aneurysm sac using lagrangian coherent structure analysis

Flow diverter (FD) stents are often used in the treatment of giant aneurysms at carotid arteries. Although these stents promise to decrease blood flow into the aneurysm sac, little is known about hemodynamics inside the aneurysm once the stent is implanted into the aneurysm neck. Porosity, effective metal surface area (EMSA), mesh density, permeability, and internal resistance factor of FD directly affect the flow diversion. This thesis includes three numerical models. In each model, the blood was modeled as a non-Newtonian fluid and a Womersley velocity profile was used as inlet condition. Mean arterial pressure is used as an outlet condition. Fluid-solid interaction (FSI) analysis was performed in a numerical model, but the effect of FSI on the analysis did not make a significant difference as a result of Finite-time Lyapunov exponent (FTLE) and increased the standard analysis time by eight times. LCS techniques were used to evaluate the time evolution mechanism of stagnation regions inside an aneurysm. The purpose of this thesis is to provide a quantitative effectiveness comparison of FD stents placed into the aneurysm neck based on their stagnation zone formations in the sac. Backward FTLE, hyperbolic time, and particle analyses were used to determine the stationary regions in the aneurysm sac. The stagnation field in the Fred FD stent implanted aneurysm was nearly 4.8 times the stagnation area of the Surpass FD stent implanted aneurysm. Besides, time-dependent results of LCS analysis show that 72- and 96-wire number Surpass FD stents are most of the blood flow is diverted into the main artery when compared with 48 wire number stent. It is noticed that use of FRED 4017 stent caused nearly 40 percent of the weightless blood flow particles (more than FRED 4038 and 4539) to stay inside the aneurysm while only 0.35 percent of the blood flow was remaining inside the aneurysm sac when no stent was placed into the aneurysm site. FTLE fields obtained from the LCS techniques demonstrated a good agreement of detecting the stagnation region with the patient's digital subtraction angiography (DSA) images obtained just after treatment.